High intensity discharge ("HID") lamps such as sodium, metal halide, mercury and others are commonly used sources of illumination due to their relatively high efficiencies in converting electrical input power into light output. It is well known that the efficiency of HID lamps is generally improved by operating such lamps with high frequency electrical input power to drive the discharge within the lamp. However, the ignition of such HID lamps brings certain associated problems due to the highly dynamic characteristics of operation of HID lamps which change from an effective open circuit when extinguished to a very low impedance (close to zero) in an extremely brief time upon ignition. Ballast circuits are required to provide electrical power to the lamp causing it to ignite and to operate efficiently following ignition. High efficiency ballast circuits have typically been quite expensive in the prior art. Ballasts delivering high voltage pulses to the lamp causing it to ignite also have the undesirable side effect of deteriorating the electrodes of the HID lamp since, under the influence of the high applied voltage, the removal of material from the electrodes is facilitated. The electrode material is generally deposited onto the interior surfaces of the lamp, leading to darkening of the lamp as well as destruction of the electrodes by such loss of material. These effects shorten the useful life of the HID lamp. A method for igniting high frequency operated HID lamps without the use of high applied voltages, which thereby avoids harmful effects on the HID lamp from the high voltage, is the subject of the present invention.
There are several methods of igniting HID lamps in the prior art. U.S. Pat. Nos. 4,724,362, 4,958,107 and 5,534,753 all use a pulse technique of ignition, applying a high voltage pulse to the lamp for the purpose of causing ionization, discharge and ignition. This ignition pulse is typically generated by first building an energy reserve in a capacitor of 1/2CV.sup.2 when capacitor having capacitance "C" is charged to a voltage "V," which is then connected to a self triggered device, such as a SIDAC or other high power switching device. When the voltage attains the device's trigger level, the SIDAC self-triggers to an "on" condition. The energy stored in the capacitor is then dumped into an inductive device, inducing an undamped ringing condition (such as U.S. Pat. No. 4,724,362) or dumped into the secondary of a transformer that up converts the energy to a high voltage pulse in order to achieve ignition of the lamp (such as U.S. Pat. Nos. 5,534,753 and 4,958,107). All of these patents generate a pulse of high voltage which carries the attendant drawbacks noted above and is avoided by the ignition method and circuitry of the present invention.
U.S. Pat. No. 4,888,528 describes a separate high frequency, high voltage oscillator (number 32 in the referenced patent) that is used to superimpose the very high frequency over whatever waveshape is otherwise applied to the lamp from the ballast (denoted by 21 in this patent). The two different frequencies, the usual lamp driving frequency and the superimposed high frequency, are segregated by inductors denoted by 27 and 28. The present invention does not require a separate high frequency generator, thus distinguishing from the invention disclosed in this patent. The present invention in one embodiment may superimpose a high frequency applied voltage onto a dc offset voltage, but does not require a separate generator for either the dc or the high frequency ac, making use of the inherent characteristics of the lamp for this purpose. Other embodiments of the present invention do not use dc, applying only ac to the lamp.
In U.S. Pat. No. 3,573,544, a secondary resonance point is used when the lamp is not ignited and capacitor (30 in this patent) resonates at the higher frequency. By increasing frequency during no load (that is, lamp extinguished), and utilizing resonate rise of voltage of a high Q circuit increases the voltage to the ignition point for that frequency, ignites the gas in the arc tube. Unlike the present invention which ignites the HID lamp at relatively low voltages, this patent uses a high voltage with a high frequency in its starting phase. The present invention is different because there is no intervention in oscillator frequency and there need not be DC offset set up on the series connected resonant capacitive element.
The method of igniting the lamp of the present invention should be distinguished from conventional ballast of the prior art. In much of the prior art, a very high voltage narrow spike is typically used to ionize the gas. The amount of energy in that very high voltage narrow spike determines how much total material gets ionized. The ionized materials have a certain latency interval which is the longest time the lamp can be unenergized and then powered again with normal voltage and still be in a conductive state. If the off time is longer than this period, the material becomes non-ionized (neutral) again and the lamp will require a cool down period to restart. If you ionize the gas material and then immediately use the main power source and sustain that arc, then the lamp will be in operation.
High voltage pulses tend to damage the electrodes in the arc tube of HID lamps. When the lamp is off, the gas is cold and the electrodes are in non-thermionic emission, which means that in order to lift an electron off the surface of the electrodes, a sufficiently strong E field must be supplied against the work function of the surface in order to lift and remove that electron. The surface of the electrode has a very strong work function, so it requires the dissipation of a lot of energy. The electrodes are typically symmetrical with sharp edges. All of the energy deposited from the typical storage capacitor, (1/2CV.sup.2) is directed towards lifting the electrons off of the electrodes and some neutral matter is also lifted off of the electrodes as well, which results in darkening or blackening around the electrode. Thus, every time a high voltage pulse is applied to the electrode of the lamp, not only electrons are removed from the electrode but other material is removed from the electrode as well. The blackening caused by material removed from electrodes also increase the opacity of a part of the arc tube which means that the light output of the lamp goes down over time. The present invention overcomes these problems in the prior art.
In addition to high voltage pulses, FIG. 1 describes qualitatively another method for igniting HID lamps. FIG. 1 illustrates the known characteristic of discharge gases that higher applied frequency will cause ignition at lower applied voltages. The effect of "hysteresis heating" is commonly presumed to cause the effect illustrated in FIG. 1 of lower ignition voltages being required at higher applied frequencies.
The hysteresis heating effect illustrated in FIG. 1 has been used in the prior art, typically in the form of a special high frequency generator constructed as part of the lamp circuit. When it is detected that the lamp is out, the high frequency generator is activated resulting in ignition of the lamp pursuant to the hysteresis heating effect of FIG. 1 This approach to ignition has the disadvantages of requiring a separate high frequency generator, and associated logic to detect when the lamp is out so the high frequency can be activated. These additional components add to the complexity and cost of the lamp ballast.
In summary, the prior art methods for igniting high intensity discharge lamps involve the application of one or more high voltage pulses, the generation and application to the lamp of high frequency, or a combination of both.
The present invention makes use of the hysteresis heating effect of FIG. 1 in combination with circuitry causing high frequencies to be applied to the lamp without the need for separate, auxiliary generation of this high frequency. The method and circuitry of the present invention automatically shifts operation to higher frequencies when the lamp is out, without the need for separate logic circuits to detect the off condition of the lamp. High voltage spikes are not required to ignite the lamp pursuant to the present invention, thus avoiding the loss of material from the electrodes, blackening of the tube, and resulting shortened lamp life. The present invention thus achieves simplified and lower cost operation of HID lamps.